\documentclass[a4paper,12pt]{article}
\usepackage{indentfirst,amsmath,amsopn,amsthm,amssymb,lscape}
\usepackage{mathtext}
\usepackage[cp1251]{inputenc}
\usepackage[english,russian]{babel}
\oddsidemargin=0cm
\topmargin=-2.5cm
\textheight=26cm
\textwidth=17cm

\def\atitle#1#2%
{\begin{center} {\bf\uppercase{#1}}\\[0.5\baselineskip] {#2}\end{center}
 \vspace{\baselineskip}\@ifnextchar[{\@atitop{#1}{#2}}{\@atit{#1}{#2}}}
\def\@atit#1#2{}
\def\@atitop#1#2[#3]{}

\newcommand{\Z}{\mathbf{Z}}
\newcommand{\Q}{\mathbf{Q}}
\newcommand{\Co}{\mathbf{C}}
\newcommand{\R}{\mathbf{R}}

 \DeclareMathOperator{\Gal}{Gal}
 \DeclareMathOperator{\Irr}{Irr}
 \DeclareMathOperator{\Norm}{Norm}
 \DeclareMathOperator{\hig}{Hig}
 \DeclareMathOperator{\X}{X}
 \DeclareMathOperator{\Un}{U}
 \DeclareMathOperator{\V}{V}
 \DeclareMathOperator{\tr}{tr}
 \DeclareMathOperator{\N}{N}
 \DeclareMathOperator{\st}{st}
 \DeclareMathOperator{\pr}{pr}
 \DeclareMathOperator{\rank}{r}


\theoremstyle{plain}
\newtheorem{theo}{\Rus Теорема}
\newtheorem{theorem}{\Rus Теорема}
\newtheorem{lemma}{\Rus Лемма}
\newtheorem{lemm}{\Rus Лемма}
\newtheorem{Zam}{\Rus Замечания}

\theoremstyle{definition}
\newtheorem{definition}{\Rus Определение}
\newtheorem{notation}{\Rus Обозначение}
\newtheorem*{notations}{\Rus Обозначения}

\theoremstyle{remark}
\newtheorem{remark}{\Rus Замечание}

\makeatletter
\renewcommand{\section}{\@startsection {section}{1}{\z@}%
                                   {-3.5ex \@plus -1ex \@minus -.2ex}%
                                   {2.3ex \@plus.2ex}%
                                   {\centering\normalfont\Large\bfseries}}
\renewcommand{\subsection}{\@startsection{subsection}{2}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\centering\normalfont\large\bfseries}}
\renewcommand{\subsubsection}{\@startsection{subsubsection}{3}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\centering\normalfont\normalsize\bfseries}}
\makeatother